1. Field of the Invention
The present invention relates to a developing solution and a method for producing a lithography printing plate. More specifically, the invention relates to a developing solution for lithography printing plate, which enables a developing treatment in a region at acidity to low alkalinity and with which development can be done even in an acidic region and which involves developing smut dispersed at good stability after development, as well as a method for producing a lithography printing plate.
2. Background Art
Generally, lithography printing plate comprises a lipophilic image part and a hydrophilic non-image part receiving ink and moistening water, respectively in a printing course. Lithography is a printing process comprising generating difference in ink deposition on the surface of a lithography printing plate by utilizing the repulsive properties of water and printing ink to each other while a lipophilic image part on the lithography printing plate is used as an ink acceptor and a hydrophilic non-image part is used as a moistening water acceptor (a part never accepting ink) to deposit ink on the image part alone and transferring the ink onto a printing matter such as paper.
So as to prepare such lithography printing plate in the related art, a lithography printing plate precursor (PS plate) with a lipophilic photosensitive resin layer (photosensitive layer, image recording layer) arranged on a hydrophilic support has been used widely. Generally, lithography printing plate is obtained by a plate-production process comprising exposing such lithography printing plate precursor through an original picture such as lith type film, subsequently leaving a part working as an image part on an image recording layer as it is, while dissolving and removing the unnecessary image recording layer except the aforementioned image part with an alkaline developing solution or an organic solvent, and then exposing the hydrophilic support surface to form a non-image part.
At the plate-production process of producing a lithography printing plate precursor in the related art, as described above, a step of dissolving and removing the unnecessary image recording layer with a developing solution and the like post-exposure is essential. Treatment with developing solutions closer to neutrality which generates less liquid waste is an issue from the standpoint of environmental and safety concerns. In view of global environment, in particular, ingredients in liquid waste discharged from wet processes draw serious concerns in the entire industries. Therefore, increasing demands are now directed toward the overcoming of the issues.
Meanwhile, recently, digital techniques for electronically processing, accumulating and outputting image information with computer have been spread widely. Various types of new image output modes corresponding to such digital techniques have been applied in practical sense. Following the trend, the computer-to-plate (CTP) technique for directly producing a lithography printing plate with no use of lith type film has drawn attention, which comprises immobilizing digitalized image information with highly collimating radiation such as laser beam and using the beam for scanning and exposing the lithography printing plate precursor. Therefore, it is one of the important technical issues to obtain a lithography printing plate precursor suitable for such technique.
As described above, the preparation of developing solutions to lower alkalinity and the simplification of the processing steps have increasingly been desired more than ever in terms of both the concerns for global environment and the conformity with space saving and low running cost. As described above, nevertheless, the developing process generally comprises the following three steps: a step of development with an alkaline solution at pH 10 or more, a step of flushing off the alkali solution in a water rinsing bath, and a step of subsequent treatment with a gum solution mainly comprising a hydrophilic resin. Therefore, an automatic developing apparatus itself occupies a significantly large space. Additionally, environmental problems and running cost problems including disposals of for example liquid waste from development, liquid waste from the water rinsing step and liquid gum waste still remain unresolved.
In contrast, for example, JP-A-2002-91016 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes a development process in an alkaline solution containing a nonionic surfactant, pH 10 to 12.5, where the development process comprises allowing a photosensitive composition to contain an alkali-soluble polymer. Therefore, the process disadvantageously involves a problem such that development cannot be attained at the above defined pH or lower. For example, additionally, Japanese Patent No. 2938397) describes a lithography printing plate precursor prepared by arranging an image forming layer with a hydrophobic thermoplastic polymer particle dispersed in a hydrophilic binder, on a hydrophilic support. Via image exposure using infrared laser, the hydrophobic thermoplastic polymer particle is thermally coupled together to form an image; then, the lithography printing plate precursor is mounted on the cylinder of a printer, and can be developed with moistening water and/or ink on the printer.
As described above, such process of forming an image via the coupling of the particles via simple thermal melting generates a great developing performance on the machine but the resulting image strength (adhesion to support) is so extremely poor, disadvantageously, that the printing resistance thereof is insufficient.
As described above, the development systems with alkaline agents disadvantageously have caused environmental concerns, the supplementation of solutions compensating the pH decrease due to carbonate gas absorption, the arrangement of an apparatus therefor, the increase of liquid waste from the systems and the running cost issue of the developing solutions.
Additionally, it is very hard to secure a developing performance via development in a region at acidity to low alkalinity and it is also difficult to disperse the components of a photosensitive layer in a non-image part stably in such developing solution. Accordingly, the components of the photosensitive layer precipitate in a developing tank. In the course of the running process, the resulting precipitates deposit as developing smut on the printing plate during the processing, disadvantageously causing image defects readily.